For a one-term president to be unseated there must be a certain amount of what politicians call a ''let's-throw-the-rascals-out'' mood in this country. People generally must be indicating they are fed up with the president or with the ruling party. It must be clear that there is a very strong and, perhaps, dominant feeling among Americans to change direction.

Weary and unhappy over the Vietnam war, the voters turned away from Lyndon Johnson and his handpicked successor, Hubert Humphrey, and elected Richard Nixon. And still blaming the Republicans for Watergate, the public voted out Gerald Ford after his brief, 21/2 years in office as Nixon's successor.

Ford's pardon of Nixon was enough to cause many voters to believe that they still needed someone else, from another party, to give them a complete break from the Nixon years.

Jimmy Carter provided a new direction, one that the voters found refreshing for a while. But he, too, was ushered out in the end by the voters after liberals in his own party, led by Ted Kennedy, had undercut his presidency by raising questions in a fiercely fought primary about Carter's capacity to govern. And Carter's failure to free the hostages before the election caused many voters to view him as weak, hence, desirably replaceable.

Q: Don't the polls show many Americans with a strongly negative view of Mr. Reagan and his way of handling the job? Isn't this the kind of anti-Reagan mood that could bode ill for Reagan in his try for reelection?

A: No doubt about it. Reagan stirs up a lot of opposition, particularly among blacks and among other minority groups. The majority of women are dissatisfied with his presidency.

That of itself would sound something like an ''I'm-fed-up-with-Reagan'' mood, enough to sweep him out of office. If it grows, it may. But as of now, this resistance to Reagan does not constitute a dominant voter view.

Most of the blacks and racial minority members who are against Reagan now never voted for him in 1980.

Many of the women now against him didn't vote for Reagan in 1980. But the anti-Reagan women vote is seen at this time to be somewhat tentative. That is, many women who raise questions about Reagan now are not so firmly ensconced in their opinion but what they might still change their minds and vote for him rather than elevate to the presidency a Democrat they trust less.

Q: But so many Americans see Reagan as a ''do-nothing'' President. Isn't this view, of itself, enough to bring about his defeat?

A: This view was indeed gaining ground until the recession began to lift. Now Reagan's critics are having difficulty in convincing the general public that the President's economic moves didn't have something to do with this recovery.

Also, Reagan's backers (and there were, of course, enough in 1980 for Reagan to score a decisive victory) are for the most part elated over what they see as a President who has been most effective.

They think he has done much to cut back on government spending and government involvement in the private sector.

And they particularly like that 25 percent cut in income tax, over a three-year period.

Q: Where is Reagan's greatest potential weakness?

A: Among the women of America. And among the blue-collar workers, many of whom voted for him last time.

But even some Democratic leaders are conceding that despite the rise in unemployment that came about under Reagan, many of the blue-collar voters are still potentially in the President's camp.

One of these leaders says his polls show that these workers still ''like the cut of Reagan.'' He says they ''still like his gutsiness'' in making and sticking to decisions.